1. Field of the Invention
The present invention basically relates to a wireless base station apparatus and a wireless communication method.
Priority is claimed on Japanese Patent Application No. 2010-016856, filed Jan. 28, 2010, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Accompanied with a recently increasing demand for wireless communication, in particular, by using a portable wireless terminal device, a technique for improving the efficiency of frequency usage has been actively performed.
In Non-Patent Document 1, IDMA (interleave division multiple access) is proposed so as to identify each channel using a plurality of channel interleave patterns. In communication based on the IDMA method, instead of a spreading process of each terminal device during transmission in CDMA (code division multiple access), a wireless base station apparatus performs user separation using individual interleave patterns assigned to communication terminals in advance. In such a case, the entire processing gain can be allocated to encoding, thereby implementing a higher degree of processing gain in comparison with CDMA.
In addition, since no spreading process is performed in each terminal device during transmission, it is necessary for a wireless base station apparatus to perform chip-level multiuser interference cancellation. Since each terminal device uses an individual interleave pattern, it is possible to approximate interference by signals transmitted from another terminal device to a Gaussian distribution. Accordingly, in comparison with a multiuser detector generally used for CDMA, a multiuser detector generally used for IDMA can perform interference cancellation with a low amount of computation.
Additionally, since multiuser reception is assumed in the IDMA communication, application of the relevant communication to uplink in transmission of a cellular (phone) system from each terminal device to a wireless base station apparatus has been widely examined In Non-Patent Document 2 relating to the application of the IDMA method to a cellular system, an individual interleave pattern is assigned to each cell so that the wireless base station apparatus can cancel interference from terminal devices belonging to an adjacent cell.
FIG. 5 is a block diagram showing the general structure of a wireless base station apparatus 9 which performs communication with a plurality of terminal devices by means of IDMA. As shown in FIG. 5, the wireless base station apparatus 9 includes an antenna 91, a receiver 92, an interference canceller 93, and decoding processors 941-1, . . . , 941-K, 942-1, . . . , and 942-N, where K and N are natural numbers.
The receiver 92 receives a signal synthesized using signals sent from a plurality of terminal devices via the antenna 91, and outputs the received signal to the interference canceller 93.
Similar to the CDMA method, the interference canceller 93 performs interference cancellation using estimated channel values (relating to channels) for the respective terminal devices and interference replica signals output from the decoding processors 941-1, . . . , 941-K, 942-1, . . . , and 942-N. In the interference cancellation, interference elements included in the received signal are cancelled, and an estimated channel value is computed for each transmitted signal. The interference canceller 93 outputs the estimated channel values to the decoding processors 941-1, . . . , 941-K, 942-1, . . . , and 942-N.
The decoding processors 941-1, . . . , 941-K, 942-1, . . . , and 942-N have the same structure, and each have a metric computation unit 95 and a log-likelihood ratio computation unit 96. In addition, the decoding processors 941-1, . . . , and 941-K in the present cell perform decoding for the channel values of terminal devices belonging to the present cell, while the decoding processors 942-1, . . . , and 942-N in the present cell perform decoding for the channel values of terminal devices belonging to the other cells.
The metric computation unit 95 computes a metric for a channel value input from the interference canceller 93. Based on the computed metric, the log-likelihood ratio computation unit 96 computes a log-likelihood ratio (LLR) and an interference replica signal, and outputs the computed interference replica signal to the interference canceller 93. The interference replica signal is computed using the log-likelihood ratio obtained in the decoding process, and fed back to the interference canceller 93.
In the wireless base station apparatus 9 having the above-described structure, the interference canceller 93 subtracts the interference replica signal fed back from each decoding processor from the received signal, thereby cancelling interference signals. Repeating such an operation improves the quality of the relevant channel value.
Additionally, the communication using IDMA employs an error correction code having a very low coding rate. Therefore, even if a symbol having a low likelihood is included in the present (target) frame, a desired likelihood can be obtained by means of the coding gain of the relevant error correction code. More specifically, the above-described feedback operation between the interference canceller 93 and the decoding processors 941-1, . . . , 941-K, 942-1, . . . , and 942-N is repeated so as to effectively cancel the interference. In addition, the quality of the signal received by the wireless base station apparatus 9 from each terminal device can be improved by increasing the number of iterations for the feedback of the interference replica signals from the decoding processors 941-1, . . . , 941-K, 942-1, . . . , and 942-N.
Due to such repetition, accuracy of the decoding result and the interference replica signal is improved, and the accuracy is further improved by using an interference replica signal whose accuracy has been improved.
Non-Patent Document 1: Li Ping et al., “Interleave-Division Multiple-Access”, IEEE Transactions on Wireless Communications, vol. 5, no. 4, pp. 938-947, April, 2006.
Non-Patent Document 2: Yunzhou Li et al., “Inter-cell Interference Mitigation based on IDMA”, IEEE C802.16m-07/115, pp. 1-6, June, 2007.
However, in the wireless base station apparatus 9 in FIG. 5, when electric power of the received signal form a terminal device belonging to another adjacent cell decreases due to attenuation through the relevant channel, a desired likelihood may not be obtained by the error correction through the repetition of the decoding process. In such a case, the interference replica signals generated by the decoding processors 941-1, . . . , 941-K, 942-1, . . . , and 942-N have low accuracy and thus low reliability, which causes a decrease in the convergence speed for the iterative operation, and also causes degradation for the convergent point of the bit error rate. Furthermore, the wireless base station apparatus 9 may degrade the quality of the signal received from a terminal device which belongs to the present cell of the wireless base station apparatus 9.